1. Field of the Invention
The present invention relates to the control of an electric generator set that includes an engine and a generator. In particular, the present invention relates to the control of an electric generator set that has a variable load at the output of the generator.
2. Description of the Related Art
Electrical generators driven by an internal combustion engine are used to provide electrical power in situations in which power is unavailable from an electric utility company. The engine-generator combination is often referred to as a “generator set” or simply a “genset.” The generator produces alternating electric current and thus often is referred to as an “alternator”. The output voltage of a genset is proportional to both the magnetic flux density within the generator, and the speed of the engine. The magnetic flux density is typically determined by controlling an armature voltage or field current of the generator, while the speed of the engine is usually determined by an engine governor.
When an electrical load is attached to the output terminals of the generator or when an attached load increases in magnitude, the speed of the engine tends to drop unless the engine governor appropriately adjusts the position of the engine throttle. In practice, adding or increasing the load does not adversely affect the performance of a genset, if the load only changes gradually or if the load is very small, at which times the engine governor is capable of effectively responding to the increased load. However, if a generator load changes too quickly, particularly if the load is large, an excessive drop in the speed of the engine can occur. In this situation, the engine governor is unable to open the throttle fast enough to maintain the engine's speed. Because the speed of the engine decreases excessively, the output voltage and frequency of the generator also decreases excessively. Such sizeable variation in the output voltage and frequency can adversely affect operation of other loads connected to the generator.
Although it would be desirable if the engine governor could respond to a speed decrease in order to maintain an engine's speed, mechanical time constraints inherent in conventional engine systems limit the rate at which a throttle can be opened.
Furthermore, while certain prior art systems exist that maintain engine speed despite sudden increases in the electrical load on the generator, none of those prior art systems both (a) maintains the voltage output level of the generator at the desired level and (b) applies to gensets in which the AC power output of the generator is directly supplied to outside power lines or other loads without rectification or inversion.
That is, one type of previous control systems maintain engine speed approximately constant by momentarily relaxing the armature voltage or field current when the load on the generator suddenly increases. By relaxing the armature voltage or field current, the effective load on the engine is decreased, and consequently the speed of the engine does not decrease as much as it otherwise would. While an excessive drop in the engine speed is prevented by these systems, the output voltage of the generator cannot remain at the desired level, but rather must decrease because of the reduction in the armature voltage or field current.
Other previous systems prevented an excessive drop in the engine speed by further opening the throttle of the engine rather than by relaxing the armature voltage or field current of the generator. The necessity to open the engine throttle further was determined by measuring DC power output of the genset. That is, these systems applied only to gensets in which the AC power output from the generators was rectified into DC power. Such gensets include rectifiers to convert the AC power into DC power, and must further include inverters to reconvert the DC power into AC power suitable for output to power lines and other AC loads. Thus, these control systems are inapplicable to gensets in which the AC power output of the generators is to be directly connected to power lines and other AC loads.
It would therefore be advantageous if another method and apparatus were developed for minimizing the transient effects on the engine speed and output voltage that occur when the genset load changes suddenly.